1. Field of the Invention
The present invention relates to an exhaust emission purification apparatus for an engine, and particularly to a technique that, in an exhaust emission purification apparatus, adds a NOx reducing agent to purify an exhaust gas, for preventing accumulation of the constituents of the reducing agent in the exhaust passage.
2. Description of the Related Art
As an apparatus for purifying nitrogen oxides (hereinafter referred to as “NOx”) contained in an exhaust gas of an engine, the following apparatus that uses a NOx reducing agent is known. The apparatus has a reduction catalyst disposed in an exhaust passage of the engine, and in the apparatus, a reducing agent such as a urea aqueous solution is injected by an injection nozzle disposed upstream of the reduction catalyst (Japanese Laid-open (Kokai) Patent Application Publication No. 2000-027627). NOx in the exhaust gas and the injected reducing agent cause a reducing reaction on the reduction catalyst, and therefore the NOx is purified into harmless constituents.
In such an apparatus, for the purpose of injection of the reducing agent, the injection nozzle is disposed to protrude through a tubular wall of the exhaust passage so as to be inserted into the exhaust passage. Therefore, a manufacturing advantage is obtained if the exhaust passage extending between the injection nozzle and the reduction catalyst is formed with an intermediate member separate from the passage member that is located upstream of the injection nozzle. Here, from the viewpoint of manufacturing economy and convenience, it is preferable to adopt a joining method in which the intermediate member is formed by welding to define an intermediate passage. Namely, a flat plate is employed as a base material of the intermediate member. The flat plate is formed by bending into a cylindrical shape and then its opposite edges are joined together by welding.
However, in the method using the welding, a weld bead must be formed in a joining portion in order to ensure joining strength. Accordingly, in the mounted state of the intermediate member in the exhaust passage, the weld bead bulges from the wall surface of the exhaust passage. If no consideration is given to the joining portion so that the joining portion is located in a lower part of the exhaust passage, the reducing agent injected by the injection nozzle is more likely to stay on the bulged weld bead while permitting the constituents of the reducing agent to be deposited on the weld bead due to the exhaust heat and consequently an accumulation of the above-mentioned constituents is formed in the exhaust passage.